The attention of EPA and state environmental agencies to assessing the vapor intrusion pathway at potentially contaminated sites is growing. On March 5, 2005, EPA discussed its pending update of its draft Vapor Intrusion Guidance (Nov 2002) at the Association for Environmental Health and Sciences (AEHS) meeting held in San Diego. Also, a vapor intrusion public seminar and regulator training hosted by the New Hampshire Department of Environmental Services in April indicated that soil gas sampling will be required at more sites.

EPA is proposing several significant changes for generic and site-specific screening. In addition, EPA intends to restructure the evaluation from a three-tiered to a four tier process.

Following is a summary of proposed changes to EPA's vapor intrusion guidance.

Tier 1 - Preliminary Screening

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This step will remain essentially the same as under the current guidance.

Tier 2 - Generic Screening

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Many of the precluding factors in the current guidance will be eliminated (e.g, presence of sewer lines).

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More realistic attenuation factors are being developed for some media. While there have been some reports that the soil gas to indoor air attenuation factor may be decreased by an order of magnitude (i.e., allowing for more attenuation between these two media), no specific values were discussed at this meeting.

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USEPA will provide non-residential screening levels in addition to the screening levels based on a residential scenario provided in the current guidance.

This step will incorporate the use of more site-specific parameters where available and will include residential and non-residential building parameters and exposure assumptions.

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Biodegradation of petroleum hydrocarbons is being considered by the agency and efforts are underway to try to address this issue within the context of evaluating the vapor intrusion pathway. In the interim, EPA is suggesting that to account for biodegradation, sample as close to the building as possible, i.e., as shallow as possible without compromising the samples by the introduction of ambient air, and to consider vertical concentration profiling including oxygen measurements.

Tier 4 - External Site-Specific Screening

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At this point in the evaluation, indoor air and/or subslab samples will be collected. The pathway will be judged incomplete if concentrations are less than the target levels.

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EPA is proposing to use of a multiple 'lines of evidence' or 'weight of evidence' approach when evaluating the site and the data to evaluate the vapor intrusion pathway, which will still include the consideration of background levels.

EPA expects to have the internal draft finalized late this summer; however, it is not clear when it will be made public. In the meantime, the draft 2002 guidance may still be used to evaluate sites with potential vapor intrusion issues, along with consideration of these potential changes.

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